type system: Add TypeEgal kind (#62001)
Now that #61915 is merged, we can start cleaning up type system
soundness issues to start, we need to separate `Type{}` (now TypeEq)
from its type equality use and the use in the cache and inference that
used `Type{}` to mean "exactly this type (by egality).
To do that, introduce a new kind `Core.TypeEgal{T}`, as an egality-based
dual of `TypeEq{T}` (`Type{T}`): its sole instance is exactly `T`
matched by `===` rather than `==`, and free typevars are disallowed
inside it. The dispatch cache now specializes type-valued arguments on
`TypeEgal` instead of `TypeEq`, so type-equal but distinct (`==` yet
`!==`) type objects receive distinct cache entries and method
specializations.
As a particular consequence, this lets us remove the unsound
`hasuniquerep` predicate that we were using to lift `Type` back to
Const. Fixes #61323.
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Co-authored-by: Keno Fischer <Keno@users.noreply.github.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>
Co-authored-by: OpenAI Codex <codex@openai.com>
